In February 2024, a resident of Deschutes County, Oregon was diagnosed with bubonic plague, most likely contracted from their pet cat. The case made national headlines, not because plague is common in Oregon, but because people were surprised plague exists in the United States at all. It does. The US averages about 7 human plague cases per year, and the bacterium that caused the Black Death in the 14th century remains permanently established in rodent populations across the American West.
Plague is not a medieval relic. It's an active zoonotic disease with a permanent wildlife reservoir that cannot be eradicated, effective treatment if caught early, and a near-certain fatality rate if missed.
The bacterium and its three forms
Plague is caused by Yersinia pestis, a gram-negative bacterium that primarily circulates among rodents - prairie dogs, ground squirrels, chipmunks, voles, and rats. It passes between rodent hosts via flea bites. Humans enter the cycle accidentally: a flea that fed on an infected rodent bites a person, or a person handles an infected animal directly.
The disease takes three clinical forms, each defined by where the bacteria establish themselves:
Bubonic plague accounts for roughly 80% of human cases. Bacteria enter through a flea bite and travel to the nearest lymph node, which swells into a painful mass called a bubo - typically in the groin, armpit, or neck. Onset occurs 2-6 days after exposure. Without treatment, bubonic plague kills 30-60% of those infected. With prompt antibiotics, fatality drops below 5%.
Septicemic plague occurs when bacteria enter the bloodstream directly (primary septicemic) or when bubonic plague progresses and bacteria spill into the blood (secondary septicemic). Symptoms include fever, chills, extreme weakness, abdominal pain, and sometimes gangrene of the extremities - the blackened tissue that gave the Black Death its name. Untreated mortality exceeds 90%.
Pneumonic plague is the form that keeps biodefense planners awake. It occurs when bacteria infect the lungs, either from inhaling respiratory droplets from another pneumonic plague patient (primary) or when bubonic or septicemic plague spreads to the lungs (secondary). This is the only form that transmits person-to-person. Untreated pneumonic plague is nearly 100% fatal, typically within 24-72 hours. Treatment must begin within 24 hours of symptom onset to be effective.
The critical difference: bubonic and septicemic plague do not spread between people. Pneumonic plague does. A single case of pneumonic plague triggers a different level of public health response, including respiratory precautions, immediate contact tracing, and prophylactic antibiotics for all contacts.
Where plague exists today
Plague is endemic on every inhabited continent except Australia and Europe. WHO reports 1,000-2,000 cases globally per year, though underreporting is likely significant.
United States. The CDC documents an average of 7 cases per year, concentrated in the rural Southwest - New Mexico, Arizona, Colorado, and California. New Mexico consistently reports the most cases. The western US acquired plague in 1900 when infected rats arrived on ships at San Francisco's port. From there, the bacterium spread through wild rodent populations across the western states and is now permanently established.
Madagascar. The global epicenter of modern plague. Madagascar reports more plague cases than any other country - typically 200-600 per year during its September-to-April plague season. In 2017, Madagascar experienced an unusually large urban outbreak centered on the capital, Antananarivo, and the port city of Toamasina. Over 2,417 confirmed, probable, and suspected cases were reported, with 209 deaths. Roughly 71% of confirmed cases were pneumonic - an abnormally high proportion that indicated significant person-to-person transmission and triggered an international WHO response.
Democratic Republic of Congo. The DRC reports the second-highest annual case count, primarily from the northeastern Ituri Province. Cases occur year-round but spike during the rainy season when rodent-flea contact with humans increases.
Peru. The primary focus for plague in the Americas outside the US, with sporadic outbreaks in the northern Andean valleys.
How transmission actually works
The classic pathway is flea-rodent-flea-human. Fleas feed on infected rodents, ingesting Y. pestis with the blood meal. The bacteria multiply in the flea's gut, eventually forming a biofilm that blocks the flea's digestive tract. The blocked flea regurgitates bacteria into the next animal it bites while trying to feed, transmitting plague with brutal efficiency.
When a rodent colony experiences a plague epizootic (an epidemic among animals), mass die-offs occur. The fleas, suddenly without rodent hosts, seek alternative blood meals - including domestic animals and humans. This is why plague risk rises after rodent die-offs, not during normal rodent activity.
Direct handling of infected animals is the other major pathway. The Oregon 2024 case likely resulted from the patient's cat, which had probably caught an infected rodent. Cats are highly susceptible to plague and can develop pneumonic plague, directly transmitting bacteria to humans through respiratory droplets or close contact. Dogs are much more resistant to plague and rarely transmit it to humans, though they can carry infected fleas home.
Hunters, trappers, and veterinarians face elevated risk. In the US, multiple cases have been linked to skinning infected rabbits, prairie dogs, or coyotes.
Treatment: effective but time-dependent
Plague is a bacterial infection. Antibiotics work. The standard treatments - streptomycin, gentamicin, doxycycline, and ciprofloxacin - are all effective against Y. pestis when started early. The antibiotics are inexpensive, widely available, and well understood.
The problem is timing. For bubonic plague, treatment within the first few days of symptoms typically results in full recovery, with mortality dropping from 30-60% to under 5%. For pneumonic plague, the window is far tighter. If antibiotics are not started within approximately 24 hours of symptom onset, mortality approaches 100%. Once pneumonic plague progresses to respiratory failure, antibiotics alone cannot reverse the damage.
This is why clinical suspicion matters. A patient in New Mexico presenting with sudden high fever, painful lymph node swelling, and a history of outdoor activity in areas with rodent populations should immediately trigger plague testing and presumptive antibiotic treatment. Waiting for lab confirmation before starting treatment can be fatal.
Post-exposure prophylaxis is standard practice. Anyone identified as a close contact of a pneumonic plague patient receives 7 days of doxycycline or ciprofloxacin. Contact tracing moves fast in plague responses - the 24-hour treatment window for pneumonic plague means every hour of delay in identifying contacts raises the risk of additional deaths.
Why plague cannot be eradicated
Some diseases can be eradicated because they have no non-human host. Smallpox only infected humans, so vaccinating enough humans ended transmission permanently. Polio similarly lacks a significant animal reservoir.
Plague is fundamentally different. Y. pestis is maintained in wild rodent populations - hundreds of species across multiple continents. You cannot vaccinate prairie dogs. You cannot eliminate every flea. The bacterium cycles through wildlife regardless of what humans do. All we can manage is the interface between the wildlife reservoir and human populations: reducing flea bites, avoiding contact with sick or dead rodents, and controlling rodent populations near human habitations.
This is the pattern with many zoonotic diseases - the pathogen's primary home is not in humans, and as long as the animal reservoir exists, spillover into human populations will continue indefinitely.
Antimicrobial resistance and biodefense
In 1995, researchers in Madagascar isolated a strain of Y. pestis carrying a plasmid that conferred resistance to multiple antibiotics, including streptomycin, tetracycline, chloramphenicol, and sulfonamides. It was the first documented case of multidrug-resistant plague. The strain was isolated from a single patient and has not been seen since, but its existence demonstrated that antimicrobial resistance in plague is possible.
If resistance to front-line antibiotics were to become widespread in Y. pestis, the consequences would be severe. The tight treatment windows for pneumonic plague leave little room for trying second-line drugs. Surveillance for resistance patterns in Y. pestis isolates is ongoing but limited by the relatively small number of cases that yield laboratory cultures.
Plague also occupies a unique position on the biodefense threat list. The CDC classifies Y. pestis as a Tier 1 Category A bioterrorism agent - the highest threat category, alongside anthrax, smallpox, and botulinum toxin. The bacterium can be grown relatively easily, aerosolized to cause pneumonic plague, and would create both medical casualties and public panic. Historical precedent exists: Japan's Unit 731 conducted plague bioweapon experiments during World War II, and the Soviet Union maintained a weaponized plague program through the Cold War.
Current biodefense preparedness includes the US Strategic National Stockpile maintaining antibiotics for plague response, though no plague vaccine is currently available for civilians. Research into next-generation plague vaccines continues, driven primarily by biodefense funding rather than natural disease burden.
Practical risk assessment
For the average person in a developed country, plague is a low-probability, high-consequence risk. You are extremely unlikely to encounter it. If you do, early recognition and treatment are the difference between a week of antibiotics and death.
If you live in or travel to the rural western US, Madagascar, or other endemic regions: avoid handling wild rodents, alive or dead. Keep pets treated with flea prevention. Don't camp near rodent burrows. If you see a cluster of dead rodents, report it to local health authorities - it may indicate a plague epizootic.
If you develop sudden fever with painful lymph node swelling after potential rodent or flea exposure, tell your doctor about the exposure explicitly. In areas where plague is rare, clinicians may not consider it without a clear exposure history. Your knowledge of the risk may be what prompts the right test and the right treatment in time.